add pybullet.changeConstraint / b3InitChangeUserConstraintCommand/ b3InitChangeUserConstraintSetPivotInB /b3InitChangeUserConstraintSetFrameInB command, to change an existing user constraint.

add constraint.py example.
allow pybullet.createConstraint to create user constraint without a child body ('fixed' to the world)

src/BulletDynamics/Featherstone/btMultiBodyPoint2Point.h

@@ -53,11 +53,12 @@ public:
 		return m_pivotInB;
 	}
 
-	void setPivotInB(const btVector3& pivotInB)
+	virtual void setPivotInB(const btVector3& pivotInB)
 	{
 		m_pivotInB = pivotInB;
 	}
 
+
 	virtual void debugDraw(class btIDebugDraw* drawer);
 
 };